Acute bronchitis is a clinical syndrome characterized by an acute, productive cough, accompanied by evidence of an upper respiratory tract infection in the absence of pneumoniae or underlying lung disease. Acute bronchitis is one of the most frequently encountered illnesses is primary care clinical practice, and treatment costs are substantial. Recent studies suggest that Mycoplasma pneumoniae is present in a significant number of patients with acute bronchitis. Randomized, controlled clinical trials of antibiotics have yielded conflicting results; appropriate treatment remains uncertain. Objectives: The objectives of this project are to: 1. Determine the utility of treatment with erythromycin in primary care patients with acute bronchitis who have no underlying lung disease. 2. Determine if M. pneumoniae is present in a significant number of primary care patients with acute bronchitis. 3. Determine if a newly developed rapid M. pneumoniae antibody test is helpful in predicting which patients will respond to erythromycin therapy. Methods: This will be a randomized, prospective, double-blind clinical trial of erythromycin in the treatment of patients with acute bronchitis. The study will be performed at three different primary care clinical sites in North Carolina. All patients will be tested for antibodies to M. pneumoniae using a newly developed rapid test as well as the traditional acute and convalescent titers by complement fixation. Patients will be seen initially and again after two weeks, with two phone call assessments to monitor side effects and compliance in the interim. Analysis of the results according to response to therapy and according to the presence of M. pneumoniae will provide information to meet the stated objectives. Sample size calculations indicate that enrolling 120 patients (40 at each site) will be sufficient.